1. Field of the Invention
The present invention generally relates to motion-vector search methods, and particularly relates to a motion-vector search method used for inter-frame predictive coding of image information.
Image-information-compression coding schemes include internationally-standardized H.261 and H.262, MPEG1 (moving picture experts group 1) and MPEG2 (moving picture experts group 2) among others. These compression coding schemes employ various compression processes such as DCT (discrete cosine transform) motion-compensation inter-frame prediction, etc. Effective enhancement of these compression processes is expected to bring about an improvement in overall compression rates of these schemes.
2. Description of the Related Art
Image-information-compression coding schemes include inter-frame predictive coding which draws on redundancy in a time dimension to achieve compression. In using this inter-frame predictive coding MPEG1 and MPEG2 can handle both a forward prediction and a backward prediction. The forward prediction detects changes between a previous frame and a current frame, and generates the current frame based on the previous frame.
FIG. 1A is an illustrative drawing for explaining the forward prediction.
As shown in FIG. 1A, picture movement between a current frame, which is to be forwardly predicted, and a reference frame (previous frame) is detected within a predetermined range defined for motion-vector search. Then, parts of the picture showing no movement are provided with zero motion vectors, and parts of the picture showing some movement are provided with corresponding motion vectors. These motion vectors are used for coding.
FIG. 1B is an illustrative drawing for explaining the motion-vector search. As shown in FIG. 1B, a given frame is divided into a plurality of blocks B.sub.1, B.sub.2, . . . Each of these blocks is separately subjected to subsequent DCT and quantization processes. In a motion-vector-search process, a given block B.sub.N of a forwardly-predicted frame is searched for in the reference frame within the motion-vector-search range. Namely, a block B'.sub.N having the highest correlation with the block B.sub.N is detected within the motion-vector-search range of the reference frame. As shown in FIG. 1B, movement of the block B.sub.N is then represented by a motion vector M.sub.N. At subsequent stages, a differential between the block B.sub.N and the block B'.sub.N is obtained by aligning these two blocks, and is subjected to the DCT and quantization processes. Results thus obtained and the motion vector M.sub.N are transmitted, stored, etc., as a part of coded pictures. The same processes are applied to all the blocks shown in FIG. 1B
In the forward prediction of FIG. 1A, the larger the motion-vector-search range, the longer the detectable motion displacement, thereby providing higher compression rates. However, a larger motion-vector-search range requires a longer processing time.
A bidirectional prediction generates the current frame based on a previous frame (past frame) and a subsequent frame (future frame). FIG. 2 is an illustrative drawing for explaining the bidirectional prediction. As shown in FIG. 2, the current frame is coded by using a backward motion vector and a forward motion vector which are obtained based on a backward reference frame and a forward reference frame. Assuming the same motion-vector-search-range size, the directional prediction has a higher prediction efficiency than does the forward prediction. Since the directional prediction suffers from twice the processing load, however, it is a general practice to reduce the size of the motion-vector-search range by half for the directional prediction.
Pictures coded based on the forward prediction are called P-pictures (predictive pictures), and pictures coded based on the directional prediction are called B-pictures bidirectionally predictive pictures). Also, pictures coded by intra-frame coding are called I-pictures (intra pictures). MPEG1 and MPEG2 employ a coding scheme which combines the intra-frame coding, the forwardly-predictive coding, and the directionally-predictive coding together, so that a picture sequence can be represented as IBBPBBPBB . . . IBBPBBPBBP . . . , for example.
In the inter-frame predictive coding of the related art, a fixed inter-frame distance is used for motion-vector search. (The inter-frame distance refers to a time distance between frames.) When an inter-frame distance is short for the forward prediction and the motion-vector-search range is relatively large, for example, some motion-vector-search process is likely to be wasted by searching in excess of a maximum range of actually existing picture motions. Also, the inter-frame predictive coding of the related art uses a fixed number of bidirectionally-predictive-coded frames. An increase in the number of these frames may result in a situation where the motion-vector-search range for the forward prediction is not sufficiently large. In this case, the prediction efficiency is decreased.
Accordingly, there is a need for a motion-vector-search method which enhances prediction efficiency by optimizing an inter-frame distance for the motion-vector search in accordance with picture characteristics.